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<title>Debugging with GDB: Type Printing API</title>

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<a name="Type-Printing-API"></a>
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<a name="Type-Printing-API-1"></a>
<h4 class="subsubsection">23.2.2.8 Type Printing API</h4>
<a name="index-type-printing-API-for-Python"></a>

<p><small>GDB</small> provides a way for Python code to customize type display.
This is mainly useful for substituting canonical typedef names for
types.
</p>
<a name="index-type-printer"></a>
<p>A <em>type printer</em> is just a Python object conforming to a certain
protocol.  A simple base class implementing the protocol is provided;
see <a href="gdb_002etypes.html#gdb_002etypes">gdb.types</a>.  A type printer must supply at least:
</p>
<dl>
<dt><a name="index-enabled-of-type_005fprinter"></a>Instance Variable of type_printer: <strong>enabled</strong></dt>
<dd><p>A boolean which is True if the printer is enabled, and False
otherwise.  This is manipulated by the <code>enable type-printer</code>
and <code>disable type-printer</code> commands.
</p></dd></dl>

<dl>
<dt><a name="index-name-of-type_005fprinter"></a>Instance Variable of type_printer: <strong>name</strong></dt>
<dd><p>The name of the type printer.  This must be a string.  This is used by
the <code>enable type-printer</code> and <code>disable type-printer</code>
commands.
</p></dd></dl>

<dl>
<dt><a name="index-instantiate-on-type_005fprinter"></a>Method on type_printer: <strong>instantiate</strong> <em>(self)</em></dt>
<dd><p>This is called by <small>GDB</small> at the start of type-printing.  It is
only called if the type printer is enabled.  This method must return a
new object that supplies a <code>recognize</code> method, as described below.
</p></dd></dl>


<p>When displaying a type, say via the <code>ptype</code> command, <small>GDB</small>
will compute a list of type recognizers.  This is done by iterating
first over the per-objfile type printers (see <a href="Objfiles-In-Python.html#Objfiles-In-Python">Objfiles In Python</a>),
followed by the per-progspace type printers (see <a href="Progspaces-In-Python.html#Progspaces-In-Python">Progspaces In Python</a>), and finally the global type printers.
</p>
<p><small>GDB</small> will call the <code>instantiate</code> method of each enabled
type printer.  If this method returns <code>None</code>, then the result is
ignored; otherwise, it is appended to the list of recognizers.
</p>
<p>Then, when <small>GDB</small> is going to display a type name, it iterates
over the list of recognizers.  For each one, it calls the recognition
function, stopping if the function returns a non-<code>None</code> value.
The recognition function is defined as:
</p>
<dl>
<dt><a name="index-recognize-on-type_005frecognizer"></a>Method on type_recognizer: <strong>recognize</strong> <em>(self, type)</em></dt>
<dd><p>If <var>type</var> is not recognized, return <code>None</code>.  Otherwise,
return a string which is to be printed as the name of <var>type</var>.
The <var>type</var> argument will be an instance of <code>gdb.Type</code>
(see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>).
</p></dd></dl>

<p><small>GDB</small> uses this two-pass approach so that type printers can
efficiently cache information without holding on to it too long.  For
example, it can be convenient to look up type information in a type
printer and hold it for a recognizer&rsquo;s lifetime; if a single pass were
done then type printers would have to make use of the event system in
order to avoid holding information that could become stale as the
inferior changed.
</p>
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